1. Field of the Invention
The present invention relates to a pneumatic tire having a tread surface which includes a plurality of grooves formed thereon and a plurality of land portions segmented by the grooves, and a rolling resistance of the pneumatic tire is reduced.
2. Description of the Related Art
Conventionally, in order to reduce a rolling resistance of a pneumatic tire, rubber of a low loss tangent ((loss tangent)=(loss modulus)/(storage modulus); hereinafter simply referred to as “tan δ”) is used as rubber disposed in a tread portion (tread rubber). However, when the rubber of low tan δ is used, although the rolling resistance can be reduced, the tread rubber tends to yield easily. As a result, a ground contact area of the tread rubber decreases resulting in a decrease of wet braking performance (braking performance on wet road surface) of a pneumatic tire. Therefore, in view of performances of a tire such as wet braking performance, it is required to reduce the rolling resistance of pneumatic tire by arranging a tread pattern while maintaining the tan δ of the tread rubber to a certain high level.
The present inventors intensively reviewed a relationship between a distortion and the rolling resistance in various portions of a tread pattern on a tire during running on the tire. As a result, it was found that the distortion was the largest at a bottom portion of a groove formed in a land portion of a tread surface in contact with a ground, and found that by reducing the distortion at this groove bottom portion, the rolling resistance of the pneumatic tire could be reduced.
Japanese Unexamined Patent Publication No. 05-338412 discloses a pneumatic tire for heavy load, which has a rib pattern/rib rug pattern formed on the tread surface. In such a pneumatic tire, a main groove has a groove bottom face having a single arc shape and a groove wall face abutting on the groove bottom face and extending being inclined in a direction that a groove width decreases toward the tread surface as viewed in a cross section of the main groove extending in a circumferential direction of the tire. With such an arrangement, this pneumatic tire is intended to prevent a convergence of distortion at the groove bottom and to thereby reduce cracking damages in the groove bottom. However, such an arrangement does not have a satisfactory effect of reducing the distortion at the groove bottom portion in the land portion. Therefore, this pneumatic tire cannot reduce the rolling resistance.
Japanese Unexamined Patent Publication No. 2001-10309 discloses a pneumatic tire having a tread surface which includes blocks. In the respective blocks, each of wall faces located at a lateral groove side is formed in a generally L-shaped recessed portion as viewed in the cross section, which expands toward the lateral groove when the block comes into contact with the ground. In the wall face of the side wall, a point of about ½ height of groove depth of the lateral groove is recessed inward the block inner than a normal line connecting a block end at the side wall and a rolling axis of the tire. With such an arrangement of the pneumatic tire, it is intended to prevent partial wear, or so-called heal and toe wear in which one end in a circumferential direction of the respective block surfaces is extremely worn into a saw-tooth like shape along a circumferential direction of the tire. However, Japanese Unexamined Patent Publication No. 2001-10309 does not suggest a method for reducing the distortion at the groove bottom portion between the land portions.